Kraken VC is obtained from defeating level 5 Kraken raid.
Duane is definitely the star of this raid event. He's a very strong dark damage dealer that can increase your overall damage since he provides a slash buff + reduce the enemy's dark resist (and slash resist at job lvl 25). This can end up making your team reach damage cap with every hit, especially if you're using a mono dark team. Definitely viable against Kraken too since it only has 15% slash resist.
Vinera and KoR Sterne are dark slashers that can reduce DEF, but KoR Sterne will hit harder since he has some light killer moves.
Shadowlynx can be a decent support unit since the single target resistance debuff will help most damage dealers even outside of dark units.
Slash units are pretty common in general, and many of them can be good against Carbuncle, but certain units can provide some good team value if they can reduce slash or DEF resist. If you have any of the other cost 100 units, then you can bring them for their debuffs, then fill the rest of your team with other damage dealers.
Zazan hits pretty hard, and he's part of the "large" raid bonus, which is already more than enough of a reason to bring him into the raid.
King Mont can hit hard, reduce slash resist, tanky, and raise his hate to avoid team damage.
Gilgamesh has slash resist and pairs well with a mono ice team if you have them so they can share VC buffs. Lasswell can reduce ice resist, and Auron just hits hard.
Omega and Behemoth has dark attack, light killer, and slash attack. Fenrir has slash + esper killer, while Leviathan has water attack + esper killer.
Sylma is a pretty good unit for Kraken, and an easy recommendation since she's free from a previous event, with enough shards to reach EX 120 too. Together with Ramuh Esper for higher magic/lightning attack + aquatic/water killer, she can hit very hard with each attack without having to buff herself by much.
Skahal is also another lightning mage that will be good with Ramuh. If you have to choose between him or Sylma, then it's better to go with Skahal since he's part of the raid bonus in the "medium" range.
Camillo and 9S are also good damage dealers for Kraken, and they're both free units. Camillo is part of the "large" raid bonus, so he can be worth dragging along even if you only have him at level 1. Simply put some offensive or defensive VC for your party and have your other damage dealers do the work.
Both Ramuh and Coeurl have already received a 3 star treatment, allowing you to get more skill board points, and both of them have water and aquatic killers + lightning/magic attack increase. It doesn't mean that you can get all of them even at 3 star max level, but it's more than enough.
There's also "Greetings from Afar" VC that gives aquatic killer party ability for more team damage.